1. Field of the Invention
The invention relates to slip-responsive devices, and more specifically to devices for shutting off driving motors when the element driven thereby experiences excessive slip.
2. Description of the Prior Art
Slipping switches are used extensively in the coal mining industry and are generally required on mine conveyor belts. Centrifugally controlled mechanical switches have been used extensively but such switches have proven unreliable and are subject to wear and damage due to improper lubrication. A switch jammed in a closed position because of a mechanical failure will not disconnect a motor when belt speed drops. An electronically activated switch avoids such problems.
A prior art attempt to solve the problems associated with using mechanical switches for conveyor belt slippage detectors is illustrated by Stiebel U.S. Pat. No. 3,845,375 which teaches the generation of pulses, used to trigger a monostable multivibrator thereby producing a sequence of pulses whose frequency is proportional to the belt speed. In operation, a capacitor is charged linearly, the voltage thereon being used to cause a unijunction transistor to fire, thereby deactivating a relay and de-energizing the motor's circuit. Stiebel, however, utilizes the interruption of a light beam to generate the pulse, and relies upon many discrete electric components which results in a rather complex circuit design.
Other prior art references of interest include Kada et al. U.S. Pat. No. 3,877,004 and Hinz U.S. Pat. No. 3,130,393. Neither of these teachings provide for automatic cut-off of the drive motor upon detection of slippage.